The present invention relates to an electrostatic pendular accelerometer sensor and to a method of controlling such a sensor. By way of example, the sensor may be a sensor of the micro electromechanical system (MEMS) type.
An electrostatic pendular accelerometer comprises a housing having a seismic mass connected thereto via one or more hinges positioned in such a manner that the seismic mass forms a pendulum that is movable relative to the housing either in translation or in rotation. The movements of the seismic mass under the effect of acceleration are generally detected by means of three electrodes. Stationary first and second electrodes are secured to the housing and connected to an exciter circuit. The third electrode is movable and is carried by the pendulum, being connected to a detector circuit.
Each stationary electrode co-operates with the movable electrode to form a capacitor of capacitance that depends on the spacing between them. In the absence of any manufacturing defects and when the sensor is not being subjected to acceleration along its sensing axis, the pendulum remains in a neutral position in which the two capacitances are equal. In contrast, when the pendulum is subjected to an acceleration along its sensing axis, it moves, thereby decreasing the capacitance formed by the movable electrode and one of the stationary electrodes, and increasing the capacitance formed by the movable electrode and the other stationary electrode. This variation in capacitances also depends on the deformations of the housing and of the pendulum.
In open-loop operation, the acceleration applied along the sensing axis of the sensor is deduced from the difference that exists between the two capacitances. This mode of operation nevertheless presents several drawbacks:                the direction of the sensing axis varies depending on the position of the pendulum when the pendulum is movable in pivoting;        there is an offset due to asymmetries in fabrication of the capacitors (electrodes of different areas and/or different gaps between the electrodes);        measurement non-linearities exist because of the way the capacitances are non-linear as a function of the movement of the electrodes;        during movements of the pendulum, the gas surrounding the pendulum compresses and expands, thereby generating forces on the pendulum;        passband is narrow because of the resonant frequency of the pendulum.        
In closed-loop operation, the position of the pendulum is servo-controlled to a neutral position or to a setpoint position, halfway between the stationary electrodes, by applying an electrostatic force to the pendulum. The electrostatic force must therefore compensate the acceleration that is applied along the sensing axis, thereby enabling the acceleration to be estimated. The electrostatic force is the result of voltages applied to the electrodes in order to keep the difference between the capacitances at zero.
The sensor has an exciter circuit for each stationary electrode that is arranged to power the electrodes so as to generate said electrostatic force.
The root means square (rms) nature of the electrostatic force relative to the applied voltages complicates the design of the control circuit serving to servo-control the pendulum and estimate acceleration. In order to work around this difficulty, it is known to apply on/off control to the pendulum using calibrated voltage pulses. The pulses are applied to one or other of the electrodes depending on whether the pendulum is to be pulled or pushed in order to be returned towards its setpoint position. The density of the pulses for pushing (or pulling) the pendulum, i.e. the number of pulses over a time interval, is then an affined function of the acceleration that is to be measured. Thus, zero acceleration is compensated by equal numbers, on average, of pulses in both directions.
Nevertheless, if the symmetry of the pulses applied to the two electrodes is imperfect (mainly because of a difference between the durations of the pulses applied respectively to the first and second stationary electrodes), the pulse density is modified by the servo-control in order to maintain the pendulum in the setpoint position, thereby biasing the estimate of the acceleration. By way of example, taking an accelerometer in which each of the voltages applied during the control stage exerts a mean force for a duration Ts=1 microsecond (μs), equivalent to an acceleration of the pendulum of 50 g, in order to maintain the bias of such an accelerometer to a value of less than 50 μg, it is necessary for the symmetry of the pulses to be controlled with an error less than the ratio 50 μg/50 g, i.e. less than 1×10−6. It is therefore necessary to control the duration of the pulses applied to each of the electrodes in such a manner that the asymmetry does not exceed 1×10−6×1 μs, 1 picosecond (μs), which is extremely difficult.
Asymmetries of implementation, which constitute the weak point of on/off control, are thus a major obstacle in obtaining better performance from such sensors.